The invention relates to a fuel injection system for internal combustion engines of the type defined herein.
In such fuel injection systems, the entire valve body of the fuel injection valve, protruding only with its mouthpiece fitting into the intake tube immediately upstream of the inlet valve in the cylinder head of the engine, is located in the immediate vicinity of the engine and thus, when the engine is being shut off, for instance, the mouthpiece fitting is exposed to pronounced thermal radiation, which also heats the fuel in the fuel injection valve. This produces vapor bubbles in the interior of the fuel injection valve. The bubbles rise and some of them are retained in the fuel filter, which with its close mesh acts as a barrier to the vapor bubbles. During this phase, evaporated fuel is thus present in the lower region of the fuel injection valve. If starting of the engine is undertaken in this hot phase (hot starting), then the evaporated fuel is sufficient for quick starting. Since the vapor bubbles in the upper region of the fuel injection valve reach the metering zone by way of the upwardly flowing fresh fuel, and the fresh fuel also evaporates at the hot surface of the valve body, the outcome is a marked reduction in the quantity of fuel injected and thus an attendant reduction in the rpm of the engine, possibly to the extent of stopping it.
In a fuel injection system (German Patent 37 05 848 Al), U.S. Patent application Ser. No. 134,718 filed Dec. 18, 1987, to avoid this disadvantage, each fuel injection valve is inserted into a cylindrical valve receptacle, which has an annular groove for inflowing fuel and, axially spaced apart from it, an annular groove for returning fuel. A radial inflow opening in the valve body connects the annular fuel inflow groove to the valve chamber, and a radial outflow opening connects the valve chamber to the annular fuel return groove. The fuel supply line coming from the fuel tank opens into the annular fuel inflow groove of the first fuel injection valve, while the annular fuel inflow groove of each further fuel injection valve communicates with the annular fuel return groove of the preceding fuel injection valve. The fuel return line that leads back to the fuel tank is connected to the annular fuel return groove of the final fuel injection valve. If "hot" starting is performed in this known fuel injection system, the fuel injection valves are rapidly and compulsorily flushed, so that any fuel vapor that may be present is flushed out of the fuel injection valves away from the valve seat, and rapid cooling of the fuel injection valves with fresh fuel assures the required fuel supply to the engine, so that the started engine continues to run unimpeded. A disadvantage of such a fuel injection system, which has so-called "side-feed" valves, is the substantially higher production price compared with the "top-feed" valves described at the outset, because of the substantially more expensive machining of the valve body that is required.